Nether Edge is right - you need to get the remuneration certificate first. There should be information on the invoice from the Solicitor about how to go about getting this, as the information has to be provided to all clients when invoicing them. I think there is a time limit on them though, so check the small print on you invoice and that should tell you what to do.
Also take seatrouts advice - just be wary of the time limit on the question of their bill.
Good luck,
Janine